Previous toxicological studies using selected symmetrical hexachlorobiphenyl isomers (representing environmentally persistent polychlorinated biphenyls (PCBs) with high and constant chlorine content) in chicks and mice have been (or will be) extended to include the one remaining untested symmetrical isomer and several unsymmetrical isomers including potential metabolite possibilities, an isomer containing the least toxic and most toxic substitution pattern in one molecule, and an isomer containing the most toxic pattern and one additional ortho chlorine affecting planarity of the molecule. Differences in isomer toxicity observed can be related to chemical structure via effects of varying chlorine substitution on compound lipophilicity and metabolism.